1. Field of the Invention
The present invention relates to a method for measuring a step difference in a semiconductor device and an apparatus for performing the same. More particularly, the invention relates to a method for measuring a step difference in a semiconductor device without making contact with the semiconductor device.
2. Description of the Related Arts
Generally, a semiconductor chip is fabricated by treating a silicon wafer through various stages of a semiconductor manufacturing process, including lithography, exposure, ion implantation, chemical and mechanical polishing, chemical or physical deposition and plasma etching. While performing the above processes, a step difference is generated in a wafer between a lower structure and an upper structure formed on the wafer. If the step difference exceeds a predetermined value, the quality of the wafer may be deteriorated.
Since present semiconductor devices have a high degree of integration, various structures or layers are continually formed on the wafer, so the step difference problem occurs frequently in conventional semiconductor manufacturing processes. Accordingly, it is essential to optimize process conditions by precisely measuring the step difference between different step areas of the wafer.
Conventionally, the step difference is measured using a sensing needle which makes contact with the stepped area of the wafer. A significant drawback to this method is that the sensing needle may cause damage to the surface of the wafer. In addition, since the sensing needle may wear out over time, the step value as measured by the sensing needle will vary, depending on the condition of the needle, Accordingly, the sensing needle must be changed periodically, which increases the cost of manufacturing the semiconductor device.
In order to solve the above problem, methods and apparatus have been proposed which purport to measure the step difference between step areas of the wafer without making contact with the wafer. One example of a non-contact measuring method is disclosed in Japanese Patent Laid-Open Publication No. 10-26515. In which a beam is radiated onto the surface of the wafer and a focusing position is detected by measuring the strength of a reflection beam so as to determine the step difference. However, since the above method detects the focusing position by differentiating a signal beam strength, it is difficult to precisely detect the focusing position, so the ability to rely on the measured value is lowered.
An example of a non-contact measuring apparatus is disclosed in U.S. Pat. No. 4,931,630, in which a focusing point is detected by sensing the strength of the interference pattern generated when a beam is radiated onto a predetermined surface of the wafer. Further, an apparatus for inspecting the pattern using scattered light is disclosed in Korea Patent Publication No. 10-0233621. While this apparatus can detect and inspect the pattern, it can not measure the step difference.